The search for new therapeutic agents has been greatly aided in recent years by a better understanding of the structure of enzymes and other biomolecules associated with diseases. One important class of enzymes that has been the subject of extensive study is protein kinases.
Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within the cell. (See, Hardie, G. and Hanks, S. The Protein Kinase Facts Book, I and II, Academic Press, San Diego, Calif.: 1995). Protein kinases are thought to have evolved from a common ancestral gene due to the conservation of their structure and catalytic function. Almost all kinases contain a similar 250-300 amino acid catalytic domain. The kinases may be categorized into families by the substrates they phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.). Sequence motifs have been identified that generally correspond to each of these kinase families (See, for example, Hanks, S. K., Hunter, T., FASEB J. 1995, 9, 576-596; Knighton et al., Science 1991, 253, 407-414; Hiles et al., Cell 1992, 70, 419-429; Kunz et al., Cell 1993, 73, 585-596; Garcia-Bustos et al., EMBO J. 1994, 13, 2352-2361).
In general, protein kinases mediate intracellular signaling by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. These phosphorylation events act as molecular on/off switches that can modulate or regulate the target protein biological function. These phosphorylation events are ultimately triggered in response to a variety of extracellular and other stimuli. Examples of such stimuli include environmental and chemical stress signals (e.g., osmotic shock, heat shock, ultraviolet radiation, bacterial endotoxin, and H2O2), cytokines (e.g., interleukin-1 (IL-1) and tumor necrosis factor α (TNF-α)), and growth factors (e.g., granulocyte macrophage-colony-stimulating factor (GM-CSF), and fibroblast growth factor (FGF)). An extracellular stimulus may affect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis, and regulation of the cell cycle.
Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events as described above. These diseases include, but are not limited to, autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease, and hormone-related diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
The Tec family of non-receptor tyrosine kinases plays a central role in signaling through antigen-receptors such as the TCR, BCR and Fcε receptors (reviewed in Miller A, et al., Current Opinion in Immunology 14; 331-340 (2002). Tec family kinases are essential for T cell activation. Three members of the Tec family, Itk, Rlk and Tec, are activated downstream of antigen receptor engagement in T cells and transmit signals to downstream effectors, including PLC-γ. Deletion of Itk in mice results in reduced T cell receptor (TCR)-induced proliferation and secretion of the cytokines IL-2, IL-4, IL-5, IL-10 and IFN-γ (Schaeffer et al, Science 284; 638-641 (1999)), Fowell et al, Immunity 11; 399-409 (1999), Schaeffer et al Nature Immunology 2, 12; 1183-1188 (2001))). The immunological symptoms of allergic asthma are attenuated in Itk−/− mice. Lung inflammation, eosinophil infiltration and mucous production are drastically reduced in Itk−/− mice in response to challenge with the allergen OVA (Mueller et al, Journal of Immunology 170: 5056-5063 (2003)). Itk has also been implicated in atopic dermatitis. This gene has been reported to be more highly expressed in peripheral blood T cells from patients with moderate and/or severe atopic dermatitis than in controls or patients with mild atopic dermatitis (Matsumoto et al, International archives of Allergy and Immunology 129; 327-340 (2002)).
Splenocytes from Rlk−/− mice secrete half the IL-2 produced by wild type animals in response to TCR engagement (Schaeffer et al, Science 284; 638-641 (1999)), while combined deletion of Itk and Rlk in mice leads to a profound inhibition of TCR-induced responses including proliferation and production of the cytokines IL-2, IL-4, IL-5 and IFN-γ (Schaeffer et al Nature Immunology 2, 12; 1183-1188 (2001)), Schaeffer et al, Science 284; 638-641 (1999)). Intracellular signaling following TCR engagement is effected in Itk/Rlk deficient T cells; inositol triphosphate production, calcium mobilization, MAP kinase activation, and activation of the transcription factors NFAT and AP-1 are all reduced (Schaeffer et al, Science 284; 638-641 (1999), Schaeffer et al Nature Immunology 2, 12; 1183-1188 (2001)).
Tec family kinases are also essential for B cell development and activation. Patients with mutations in Btk have a profound block in B cell development, resulting in the almost complete absence of B lymphocytes and plasma cells, severely reduced Ig levels and a profound inhibition of humoral response to recall antigens (reviewed in Vihinen et al Frontiers in Bioscience 5:d917-928). Mice deficient in Btk also have a reduced number of peripheral B cells and greatly decreased levels of IgM and IgG3. Btk deletion in mice has a profound effect on B cell proliferation induced by anti-IgM, and inhibits immune responses to thymus-independent type II antigens (Ellmeier et al, J Exp Med 192:1611-1623 (2000)).
Tec kinases also play a role in mast cell activation through the high-affinity IgE receptor (FcεRI). Itk and Btk are expressed in mast cells and are activated by FcεRI cross-linking (Kawakami et al, Journal of Immunology; 3556-3562 (1995)). Btk deficient murine mast cells have reduced degranulation and decreased production of proinflammatory cytokines following FcεRI cross-linking (Kawakami et al. Journal of leukocyte biology 65:286-290). Btk deficiency also results in a decrease of macrophage effector functions (Mukhopadhyay et al, Journal of Immunology; 168, 2914-2921 (2002)).
Accordingly, there is a great need to develop compounds useful as inhibitors of protein kinases. In particular, it would be desirable to develop compounds that are useful as inhibitors of Tec family (e.g., Tec, Btk, Itk/Emt/Tsk, Bmx, Txk/Rlk) protein kinases, particularly given the inadequate treatments currently available for the majority of the disorders implicated in their activation.